Every quote in a market lands on a grid. Prices can only be displayed at certain increments, orders below a certain size don’t count as a standard unit, and exchanges charge or pay for the act of executing against a quote.
Those three parameters shape the spread a trader pays, where liquidity accumulates in the book, and how routing algorithms behave. They’re also set by rule rather than by the market, which means they change when the rules change.
The US grid has just been redrawn, and the effects show up on the screen rather than in the regulatory text.
Why Microstructure Belongs in a Trader’s Education
Analysis gets most of the attention in trading education, which makes sense. Working out what an asset is worth is the part that feels like the job.
An investment learning platform that covers structure alongside analysis fills in something screen time supplies only slowly. Structure explains why identical strategies produce different results in different instruments, why a spread widens without any news, and why an order that should have filled cleanly didn’t.
None of that appears in a chart. It sits in the mechanics underneath, and the mechanics are documented and readable if a trader knows to look.
The Half-Penny Tick and What It Covers
For two decades, US stocks priced above a dollar could only be quoted in whole cents. That created a floor under the spread in heavily traded names, where a one-cent gap was wider than genuine supply and demand would otherwise support.
The rule now allows a finer increment for those securities. Regulators estimated that the half-penny tick would apply to almost 1,800 stocks, accounting for approximately 66% of share volume and 43% of dollar volume.
Which stocks qualify isn’t fixed. Exchanges measure a time-weighted average quoted spread over a three-month window twice a year, and the resulting increment applies for the following six months. A stock can move between tiers as its trading characteristics change.
For a trader, the practical consequence is a narrower minimum spread in the most liquid names, alongside more price levels in the book and potentially smaller size resting at each one.
Access Fees, Rebates and Routing Behaviour
The second change is less visible and arguably more consequential.
Exchanges have long charged a fee to execute against a displayed quote and paid a rebate to the participant who posted it. That fee cap has come down sharply, from 30 mils to 10 mils across the board, a reduction expected to drive participant behaviour through the correspondingly lower implied rebate.
Rebates influence where orders get routed. A smaller rebate changes the economics of posting liquidity on an exchange versus executing elsewhere, which affects how much displayed depth appears on lit venues.
The same coverage notes that the rules survived a legal challenge in October 2025, though industry views on the final calibration were not unanimous.
Round Lots and Odd-Lot Visibility
A quieter change affects what gets displayed at all.
The standard round lot has historically been 100 shares, which works poorly for stocks trading at several hundred or several thousand dollars. Orders below that size, known as odd lots, haven’t appeared in the public quote even when priced better than the displayed best bid or offer.
The round lot definition now varies by price level, and the industry is building the infrastructure to disseminate the best odd-lot orders. For expensive stocks, that means a meaningful amount of previously invisible pricing becomes visible.
What Changes at the Screen
For an active trader, a few things follow:
- Minimum spreads narrow in tick-constrained names, lowering the cost of crossing
- Displayed size per price level may shrink, since liquidity spreads across more increments
- Depth reads differently, and a book that looks thinner may hold the same total size
- Quoted prices in expensive stocks improve as odd-lot orders become visible
- Tier assignments change twice a year, so an instrument’s increment isn’t permanent
The third point is the one most likely to cause misreadings. A book with more levels and less size at each isn’t necessarily less liquid than one with fewer, larger levels. It’s differently arranged, and comparing depth across the transition requires accounting for that.
What to Watch From Here
Regulators have signalled interest in studying what actually happened: how market quality responded, how quoted spreads reacted to the lower fee cap, and whether order flow shifted between exchange and off-exchange venues.
That last question is the interesting one. Changing the economics of posting on an exchange while off-exchange venues operate under different rules could push flow in either direction, and the answer isn’t obvious in advance.
For traders, the useful posture is empirical rather than predictive. The instruments they trade have measurable spreads and measurable depth, both before and after. Checking what changed in their own names is more informative than any general forecast about where the market ends up.